The ocean floor, which covers nearly three-quarters of Earth's surface, was created at spreading ridges. Since 1968, the Ocean Drilling Program (and its predecessor, the Deep Sea Drilling Project) has explored this vast and forbidding terrain. Scientists have retrieved thousands of precious samples from beneath the sea floor — the most inaccessible part of Earth’s surface. No single drill hole has completely penetrated the oceanic crust. However, using samples collected from widespread sites, scientists have pieced together a cross section from the sea floor to the upper mantle.

Layer 1 – Sediment
Although deep-sea ooze looks like clay, the microscope photo reveals that it is made mostly
of shells of tiny marine organisms. The shells record short- and long-term changes in Earth's climate.

Layer 2 – Basalt
These three samples reveal
the volcanic origins of oceanic crust. The pillow basalt erupted and cooled quickly on the sea floor.
The mineralized basalt was metamorphosed by hot
water near a black smoker. The basaltic dike was
injected into solid basalt.

Layer 3 – Gabbro
Much of the magma that feeds spreading ridges stagnates near the base of the oceanic crust, where it crystallizes to form the coarse-grained plutonic rock called gabbro. The large, interlocking crystals are evidence that it cooled slowly.

Layer 4 – The Mantle
Rising basaltic magma leaves behind the dense mantle rock called harzburgite. When minerals in harzburgite are replaced by less-dense serpentine, large masses of this altered rock may ascend to within reach of the drill.

In 1993, the Ocean Drilling Program made its deepest penetration of the sea floor at Hole 504B in the eastern Pacific Ocean. The drill was suspended 3,469 m (11,381 ft) through the ocean before it penetrated 2,111 m
(6,926 ft) of rock a total of more than 5.5 km (3.5 mi)!